1. Field of the Invention
The present invention relates to a method of preventing amyloid plaque formation and/or growth by reacting amyloid peptides with an enzyme that recognizes amyloid peptides, and inactivates them. The present invention also relates to a method of treating Alzheimer's disease by either administering an amyloid peptide degrading enzyme while minimizing or eliminating toxic side effects associated with amyloid peptide byproducts or by increasing the synthesis of the enzyme by administration of pharmacological agents that regulate the expression of the amyloid peptide degrading enzyme or by increasing the activity of the enzyme by administration of pharamacological agents.
2. Brief Description of the Related Art
Considerable effort has been expended in identifying the beta and gamma secretases that process the amyloid precursor protein to form the Aβ peptides. The goal of such studies has been to develop specific inhibitors of these enzymes in the hope that such compounds would inhibit the formation of amyloid plaques. The recent report of an aspartyl protease, which appears to be a true beta secretase (R. Vassar et al. (1999) Science 286, 735-741), provides optimism that this approach can soon be tested.
An alternative strategy is to hydrolyze Aβ peptides before they form amyloid plaques or at least prevent the further development of existing plaques. It may also be possible to remove existing plaques by hydrolyzing any plaque derived Aβ peptide in equilibrium with free Aβ peptide. We test this approach using the zinc metallopeptidases insulysin (also referred to as insulin degrading enzyme, IDE, EC. 3.4.22.11) and neprilysin (also known to as endopeptidase 24.11, NEP, CALLA), although other peptidases such as endopeptidase 24.15, endopeptidase 24.16, endothelin converting enzyme and angiotensin converting enzyme can be employed. There are a number of reasons to using insulysin and neprilysin for this purpose. First, as noted below, both insulysin and neprilysin cleave Aβ1-40 and Aβ1-42 into what appears to be innocuous products. Second, both insulysin and neprilysin are a true peptidase in that they do not hydrolyze proteins. The enzymes cleave a limited number of peptides in vitro including insulin and insulin related peptides, β endorphin, enkephalins, substance P and Aβpeptides. Third, cell surface forms of insulysin and neprilysin have been described as well as a secreted form of insulysin. Lastly, insulysin and neprilysin have been suggested to be physiological Aβ metabolizing enzymes.
Kurichkin and Goto (I. V. Kurochkin and S. Gato (1994) FEBS Lett. 345, 33-37) first reported that insulysin enzyme can hydrolyze Aβ1-40. This finding was confirmed in two separate studies (W. Q. Qul et al. (1998) J. Biol. Chem. 273, 32730-32738; and J. R. McDermott and A. M. Gibson (1997) Neurochem. Res. 22, 49-56); one of these (W. Q. Qui et al. (1998) J. Biol. Chem. 273, 32730-32738) was a collaboration with the applicant/inventor. Selkoe has proposed that insulysin could play a role in determining Aβ peptide levels after their secretion from neuronal and microglial cells (K. Vekrellis et al. (1999) Soc. For Neurosci Abstracts 25, 302). It was suggested that factors that reduce insulysin activity, i.e. oxidative damage, can lead to decreased Aβ metabolism and increased amyloid deposits (I V. Kurochkin and S. Gato (1994) FEBS Lett. 345, 33-37). Although these studies demonstrated that insulysin can hydrolyze Aβ1-40, they involved the use of either partially purified enzyme preparations such that the products of the reaction could have arisen from secondary cleavages by contaminating peptidases (I. V. Kurochkin and S. Gato (1994) FEBS Lett. 345, 33-37; and J. R. McDermott and A. M. Gibson (1997) Neurochem. Res. 22, 49-56), or the reaction products were not identified (I. V. Kurochkin and S. Gato (1994) FEBS Lett. 345, 33-37). Furthermore, it was not determined whether the products of insulysin action on Aβ1-40 are neurotoxic or could contribute to amyloid plaque formation, and Aβ1-42 was not tested as a substrate.
Howell et al (S. Howell, J. Nalbantogluand and P. Crine, Peptides (1995), 16 647-652) first showed that neprilysin could hydrolyze Aβ1-40.
Thus, there is a need in the art for a method of preventing amyloid plaque deposition and methods for treating Alzheimer's disease while minimizing toxic side effects.
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